The inheritance of variability in the behavioral and physiologic effects of d-amphetamine is being studied. A small dose of amphetamine (0.3 mgm/kg) is administered to normal volunteer twins after medical and psychiatric screening. Preliminary data suggests stable and heritable responses in: 1) behavioral ratings and 2) pharmacokinetics of amphetamine. An increase in serum prolactin is found in some individuals, and appears heritable. Pulse and blood pressure and subjective assessment of mood by a self-rating scale were found to be stable in individuals but not heritable. Nocturnal motor activity seems to be heritable but not stable in an individual. A twin study of rapid eye movement (REM) sleep induction response to Arecoline is also underway.